The invention is concerned with a process for producing regular porous pearl celluloses having a particle size in the range from 2 to 1,000 xcexcm. Furthermore, the invention is concerned with a pearl cellulose with specific properties as well as the use of the pearl cellulose with specific properties as well as the use of the pearl celluloses produced according to the process of the invention.
Regular porous pearly celluloses are a relatively inexpensive, stable material with variably adjustable chemical properties compared with other separator and carier materials. To an increasing extent cellulosic molded articles are gaining significence as chromatographic material, carriers for enzymes, cells and other ligands, e.g. after activation and coupling of proteins.
The known processes for producing such molded cellulose articles differ substantially from each other by the type of the cellulose material used, the solvent used, the manner of coagulation or regeneration, as well as the technology of division.
Thus, the patent rights JP 48-2173, JP 48-60753, JP 62-191033, Cs 172 640, U.S. Pat. No. 2,543,928, DE 2 005 408, etc., specify the use of alkaline solutions of cellulose xanthate (viscose) which are either sprayed into an acid precipitating bath or regenerated by an acid or thermal decomposition after having been dispersed in a solvent immiscible with water. A disadvantage of the procedure is that a considerable danger potential for the environment is associated with the sulfur compounds released with the regeneration, the resulting dilute acids and salt solutions and by the used organic solvents, respectively.
Other processes, e.g. according to DD 259 533, propose the use of solutions of cellulose carbamate. A specific disadvantage of this process is the necessity of an expensive after-treatment in which urea must be removed with hot water and residual carbamate groups must be decomposed with soda lye.
According to a further group of protective rights one of them from highly substituted organo-soluble cellulose esters among which cellulose acetate with an average substitution degree (DS) between 2 and 3 is preferably used. The principle of these processes passing via pearls of cellulose acetate as an intermediate results in that cellulose acetate is preferably dissolved in a halogenated hydrocarbon, the polymer solution is dispersed and solidified by vaporisation of the solvent. After separating the cellulose acetate particles the acetate groups are usually split off by a treatment with sodium hydroxide solution, e.g. JP 53-7759. As with this procedure only particles with a low porosity are obtained, many processes have been proposed which aim at a higher porosity of the resulting molded cellulose bodies. The chosen method is the addition of various pore forming agents to the cellulose acetate solution. The patents JP 56-24429, JP 24430, JP 62-267339, JP 63-68645 and U.S. Pat. No. 4,312,980 propose the use of linear alcohols. Motozato et al, J. Chromatogr. 298 (3), (1984) 499-507 prefer for this purpose hydrocarbons, such as hexane, cyclohexane, petroleum ether, toluene and similar compounds. Furthermore the patent JP 63-68645 proposes the use of long-chain carboxylic acids or esters of carboxylic acids for this aim. With all of these modifications the disadvantage is the necessity to use toxic halogenated hydrocarbons as the solvent.
The process of the patents SU 931 727 and SU 1 031 966 the subject of which is the production of cellulose pearls from cellulose acetate with a DS of 2 from a mixture of ethylacetate and n-butanol allows no adjustment of porosities of  less than 75%. The proposed use of oleic acid requires additional washing processes with use of volatile organic solvents.
A procedure for producing pearly cellulose particles with use of cellulose silylethers as specified in the patent DD 295 861 also uses volatile hydrocarbons or toxic halogen hydrocarbons as the solvent. With the acid or alkaline regeneration substantial amounts of silyl side groups remain behind on the cellulose which groups substantially restrict the use for chromatographic and medical purposes.
Up to now for the direct cellulose dissolution solvents have been proposed which are difficult to handle. Thus, the patents DE 1 792 230, FR 1 575 419, U.S. Pat. No. 3,597,350 specify the use of cuoxam and similar compounds.
The protective right JP 80-44312 and Kuga, J. Chromatogr. 195, (1980), 221-230 propose working in melts of CaSCN.
Furthermore, in JP 82-159802 mixtures of dimethylsulfoxide and paraformaldehyde are specified as solvents. Especially the multi-component solvents cause substantial problems when introducing celluloses of higher molecular weight in amounts above 5%. In addition these solvent mixtures can be recycled only to a very restricted extent.
With regard to dispersing the polymer solution after having left the nozzle in principle three technologies have been specified. The patents U.S. Pat. No. 5,047,180 and U.S. Pat. No. 5,328,603 teach the production of spherical molded pieces by spraying (atomizing) a polymer solution. In the last of said patents the multicomponent solvent dimethylacetamide/LiCl is used as a solvent for cellulose. Such a system requires salt additions of more than 10% for the manufacture of regular particles. The EP 0 268 866 realizes the division into polymer droplets by superposition of the longitudinal motion of the polymer solution exiting from the nozzle by a rotating vibration motion. Finally in DE 44 24 998 spherical particles are produced by dividing a polymer solution exiting from the nozzle by means of extremely thin rotating cutters. All the process variants are identical in that an irreversible coagulation step follows directly after dividing the polymer solution. With this it is necessary that the polymer particles adopt the regular shape when passing a more or less short distance of falling. This results in problems with forming an ideal spherical shape by premature hardening, deformations as a result of the impact on the surrounding collecting cylinder and possibly gluey coatings on the cutters so that more or less distinct variations of the shape must be accepted.
It is an object of the invention to provide a process for producing regular porous cellulose pearls which is technically simple and economical and allows the production of pearl bodies having a defined particle diameter with a narrow particle size distribution in the overall range from 2 to 1,000 xcexcm and with a broad range of variation of the adjustable porosities. Especially the process should allow to produce cellulose pearls with particle sizes in the partial range from 2 to 50 xcexcm or in the partial range from 40 to 1,000 xcexcm. With this process salt-free solvents, particularly one-component solvents are to be used which are little toxic or non-toxic. A further object is to provide a process in which the indicated drawbacks of the known processes are avoided. Finally it is the object of the invention to provide a new pearly cellulose with new applications. Further advantages can be gathered from the following specification.
With the process mentioned at the beginning these objects are achieved according to the invention in that
a) a cellulose having a degree of polymerisation in the range from 150 to 2,000 is dissolved in a solvent to form a solution of 0.5 to 25% by mass cellulose,
b) the cellulose solution is finely divided and dispersed in a dispersant which is not miscible with said solution and has a viscosity in the range from 10 to 80,000 mpaxc2x7s,
c) the dispersed solution particles are solidified to regular pearl particles by precipitating with a liquid precipitating agent miscible with the solvent
1) after cooling the dispersion to below the melting temperature of the cellulose solution and separating the frozen particles of the cellulose solution from the dispersant or
2) directly in the dispersion, and
d) the pearl particles are separated from the liquid mixture of solvent, precipitating agent and possibly dispersant.
step a) may include other than cellulose particles at least one inert solid introduced in an amount ranging from 5 to 200% by weight relative to the cellulose present. The inert solid can be a powdery material with particle size diameters ranging from 50 to 3,000 xcexcm. More specifically, the inert powdery material, other than cellulose, may have particle size diameters of greater than 40 xcexcm. The particles may be polysaccharides or inorganic compounds.
Step b) can be performed with a volume ratio of cellulose solution to dispersant in a range from 1:1 to 1:20.
with a preferred embodiment of the process for preparing pearly celluloses with a particle size in the range from 2 to 50 xcexcm the solution with 0.5 to 15% by mass cellulose is dispersed directly in a liquid inert medium, and the dispersion is further processed by the procedure of the steps c) and d). It has been found that the separation of the steps of forming and solidification by converting the cellulose dispersion after lowering the temperature into a suspension results in substantial simplifications of the production process.
With another preferred embodiment of the process of the invention for preparing pearl celluloses with a particle size ranging from 50 to 1,000 um the cellulose solution is shaped in the step b) by pressure to form at least one strand with a diameter in the range from 40 to 1,000 um, the solution strand is divided by rotating cutter jets into defined sections, and these solution particles are collected in the dispersant and maintained in motion. The separation of the process steps of dividing, forming and solidifying results in a simple manner in highly regular pearl cellulose of a narrow particle size distribution and in variably adjustable pore volumes and allows a substantial simplification of the production process.
Further embodiments of the process of the invention are defined by separating the phases of the suspension, the inert medium for division or dispersion which is immiscible with water, can be recycled directly, i.e. without additional purifying steps, such as extraction etc., into the process cycle and can be reused for the shaping process, possibly after having added auxiliary agents for enhancing the dispersing procedure.
The frozen polymer droplets separated by filtration or centrifugation can be advantageously solidified in a precipitating bath with maintenance of the properties achieved in the shaping step. The separated mixture of solvent and precipitating agent can be separated e.g. by use of thermal energy or by membranes so that the onecomponent-solvent can advantageously circulate in a short cycle.
The concentration of the cellulose solution can be adjusted within broad limits as a function of the desired particle size or pore volumes whereby solutions with a cellulose concentration from 0.5 to 15, preferably 1 to 12 and especially 2 to 7% by mass are suitable for producing the regular porous cellulose of the invention having particle diameters from 2 to 50 xcexcm and 50 to 1,000 xcexcm, respectively.
The technique without a rotating cutter used with the production of pearly cellulose in the size range from 50 to 1,000 xcexcm minimizes the danger of glueing the cutting tools with polymer droplets and additionally attributes to the high particle uniformity. For equalizing the particles and forming homogeneous particle sizes it is helpful to maintain the division during and/or after the introduction of the cellulose droplets in the dispersing medium until a stable suspension is formed by the precipitation and reduction of temperature, respectively.
When forming particles in the range from 2 to 50 xcexcm viscosity gradients between the cellulose solution and the dispersion medium are utilized. For equalizing the particles and forming small particle sizes rapidly rotating dispersing devices, for example of the type Ultra-Turrax with a speed in the range from 1,000 to 10,000 minxe2x88x921 with intensively acting dispersing tools, for example with a cutter-mixer-head or with dispersing bars, are used during and/or after charging the dispersing medium with the cellulose solution. Subsequently the division/dispersion can be maintained until a stable suspension is formed by lowering the temperature.
The possibly frozen polymer droplets are separated by filtration or centrifugation and then supplied to a precipitating bath in which they are solidified. After having separated the obtained regular porous cellulose pearls from the precipitating bath by filtration or centrifugation a washing or purification step with water or lower alcohols is carried out in the temperature range from 3 to 90xc2x0 C.
Conveniently emulsifiers such as non-ionic surfactants from the group of polyoxyethylene alkylether, polyoxyethylene arylalkyl-ether or polyoxyethylene sorbitan alkylether, are added to the inert division or dispersion medium.
The pearls celluloses can be subsequently activated and possibly coupled with various ligands via spacers. If necessary they are dried. The pearl celluloses of the invention are characterized by a particle size range from 50 to 1,000 xcexcm, a pore volume from 5 to 95% and an exclusion limit xe2x89xa65xc3x97106 Dalton, or they are characterized by a particle size range from 2 to 50 um, a pore volume  less than 50% and an exclusion limit xe2x89xa65xc3x97104 Dalton. These properties can be adjusted within narrow tolerances in the specified ranges by the process of the invention and the respective embodiments of the process. The pore volume indicates the portion of a cellulose sphere which is characterized by more or less big voids. The pore volume can be determined by electron microscopy of different sections of the cellulose sphere, by Hg-porosimetry or with a known dependency by the water retentionxe2x80x94CRC-value (DIN 53814).
The pearl celluloses of the invention, especially those with a particle size ranging from 2 to 50 xcexcm, a pore volume of less than 50% and an exclusion limit of xe2x89xa65xc3x97104 Dalton can advantageously be used for example as separating agent and carrier agent for chromato-graphic and diagnostic purposes, e.g. for diagnostic agents and bio-catalysts, as selective or specific adsorbent with the detoxification of blood and as a cell culture carrier in biotechnology, biomedicine and medicine. The pearl cellulose is particularly useful as a matrix for the gel filtration chromatography (GFC) by which molecules, mainly macromolecules are separated on the basis of the pore diameter and the exclusion limit, respectively.
The exclusion limit characterizes the limit of the magnitude of a void (pore) up to which a molecule can penetrate into this void, even only partially. It represents therefore the greatest possible dimension of a molecule for which a chromatographic separation is still possible. The exclusion limit is determined by measuring the permeation of known substances with a defined molecular size. In the present case the exclusion limits are determined by the permeation of high-molecular Dextran Blue (compare J. Baldrian et al: xe2x80x9cSmall-angle scattering from macroporous polymers: styrene divinylbenzene copolymers, cellulose in bead formxe2x80x9d in Coll. Chechoslov. Chem. Commun. 41 (1976) 12, p. 3555-3562). Particle size, pore volume and exclusion limit are always used in their entirety by producers and users in order to characterize the different products and to compare them.
The here-above used term xe2x80x9cregularxe2x80x9d means uniform in the sense of uniformity of the geometric shape. Ideally formed spheres allow an optimum packing density (hexagonally most closely packed structure). With chromatographic separation processes, good flow conditions for the phase to be separated and a good mechanical stability of the packing can be achieved with xe2x80x9cregularxe2x80x9d pearls or beads. As a result closely distributed separation curves are detected. The use of